Recently, photo degrading organic waste material is a hot topic. Many photocatalysts based on semiconductor oxide have been reported. Although they have excellent catalytic activity under ultraviolet (UV) irradiation, it is difficult to practical application because of the limited ratio of UV light in whole solar irradiation. In this work, we prepared solar driven photocatalyst based on graphene quantum dots (GQDs) and silver nanoparticles (Ag NPs) by simple photo-reduction reaction. In the synthesized process, GQDs were served as reducing agent for Ag cations and capping agent for the in situ formation of Ag NPs with small size. Although the used concentration of Ag cations was very low (just 0.54 mg/ml), GQDs–Ag photocatalyst exhibited a high efficiency for photo degradation Rhodamine B under sunlight. It could be ascribed to the synthetic role of Ag NPs and GQDs, and further the detailed photocatalytic mechanism was analyzed in this paper. Because this kind of photocatalyst made from water solution and can be driven by sunlight, it is a promising photocatalyst for industrial applications in the environmental remediation field.

In this work, phosphorus-doped graphene quantum dots (P-GQDs) with a high phosphorus doping content (>7 at%) are synthesized via an electrochemical approach. Sodium phytate (C6H6Na12O24P6), a green food antioxidant additive, is used as the electrolyte for providing both a phosphorus source and an electrolysis environment. The obtained P-GQDs exhibit excellent scavenging activity of free radicals, such as hydroxyl radicals (˙OH) and 2,2-diphenyl-1-picrylhydrazyl (DPPH). Combined with Raman, FT-IR, and XPS spectral analyses, the reason for high phosphorus content and the mechanism of free radical scavenging of P-GQDs are investigated in our work.

•The Ag-GQDs compounds were successfully prepared via photochemical reaction.•The Ag-GQDs compounds were then collected in a SiO2 microspheres-actived substrate.•GQDs with the size of 1–4 nm in Ag-GQDs compounds can act as “hot spot” sites.•GQDs favor the adsorption of Rhodamine 6G molecules and the charge transfer process.•The SiO2 template provides a larger surface areas to adsorb more probe molecules.Here, silica (SiO2) supported silver nano-particles (Ag NPs) and graphene quantum dots (GQDs) compounds were designed and synthesized via a “green” photochemical approach and an electrophoresis deposition technique in order to provide a highly active surface-enhanced Raman scattering (SERS) substrate. In this approach, the electrochemically prepared aqueous solution of GQDs was used as a solvent and a reducing agent to synthesize in-situ Ag-GQDs compounds under Ultraviolet (UV) irradiation. These compounds were collected on a SiO2 supported Si substrate through the electrophoresis deposition technique. Benefiting from their proper size (1–4 nm) and distribution in the spatial gaps between adjacent Ag NPs, GQDs could act as “hot spot” sites for lighting up the Raman scattering signals. Together with the enhanced adsorption of Rhodamine 6G (R6G) molecules through π-π stacking, the electrostatic interactions from GQDs, and the enlarged specific surface area provided by the SiO2 template, the as-prepared substrate exhibited a strong SERS signal with excellent reproducibility. The detection limit of R6G was pushed to 8.0 × 10−14 M. We hope our work provides a time-saving and facile approach for the design and creation of ultrasensitive SERS substrate for trace species detection.Download high-res image (238KB)Download full-size image

•Size controllable graphitic quantum dots were prepared via an electrochemical route.•They show identical PL spectra at 445 nm on being excited by 360 nm.•The PL emission originates from the surface oxygen groups rather than sizes.To control the size of graphitic quantum dots (GQDs), an electrochemical method was successfully developed with applied potentials cycling between −5.0 and 5.0 V at various scan rates from 0.2 to 0.5 V/s. The average size of the resultant GQDs decreases from 12 to 3.4 nm with increasing scan rates, as verified by their transmission electron microstructure (TEM) images. The X-ray diffraction (XRD), optical absorption, photoluminescence (PL), Fourier Transform Infrared (FTIR) and photoluminescence excitation (PLE) behaviors of those GQDs were investigated. The result shows that the n–π* electronic transition resulting from the surface oxygen groups contributes to the PL emission, besides the π–π* electronic transition due to their sizes.

Graphene quantum dots (GQDs) have attracted considerable attention because of their unique photoluminescence (PL) properties. Nowadays, several approaches have been reported to improve PL quantum yield (PLQY) and regulate PL colors for their different applications. However, most reports show that higher oxygen content leads to lower PLQY. Here, we report a novel approach to enhance PLQY at high oxygen content. Both oxidation and reduction of GQDs have been demonstrated to improve the PLQY of GQDs and control their PL colors after they were first prepared. The oxidation treatment using hydrogen peroxide (H2O2) and ultraviolet (UV) light, enhanced the PLQY of GQDs from 1.51 ± 0.02% to 3.99 ± 0.02%, and the PL color could also be tuned from green to yellow-green under UV irradiation. When the UV light was removed, reduction reaction occurred immediately, which further improved the PLQY to 10.37 ± 0.01% and changed the PL color to blue. Since there was no heteroatom introduced and the GQDs maintained their original size and concentration, these treated GQDs allow us to combine the detailed structural and optical studies to testify the chemical nature of the observed PL: the PL originated from different surface states and the specific hybridization of states from the surface functional groups and the connected graphene core is responsible to specific PL colors.

This work reports a modified electrochemical method for rapid and large-scale preparing graphene quantum dots (GQDs) by introduction of active free radicals, which were produced by hydrogen peroxide or ultraviolet radiation. These free radicals can deepen the oxidized or reduced level of working electrode in electrochemical process and thus lead to GQDs with high concentration and small size, but different surface oxidized degree. The improved oxidation and reduction mechanism were analyzed in this work. Meanwhile, the optical properties and oxidizability of GQDs with different surface oxidized degree were investigated. It is found that these GQDs can be used as an oxidizing agent and their oxidizability is related to the degree being oxidized.

Herein, we reported a simple and “green” method for preparing the ternary photocatalyst Ag–graphene quantum dots (GQDs)–ZnO. In this method, an aqueous solution of GQDs not only acted as a substituent for the organic solvent for preparing the ZnO precursor but was also used as a reducing agent for the in situ synthesis of Ag nanoparticles (NPs). X-ray diffraction analysis and scanning electron microscopy were employed to confirm the effects of the GQD solution as a solvent on the ZnO structure. Transmission electron microscopy confirmed the synthesis of Ag NPs in the GQD solution as well as the formation of close interconnections between them. Furthermore, photocatalytic tests involving the degradation of Rhodamine B showed that the synthesized ternary photocatalyst displayed excellent visible-light photocatalytic activity, which was much higher than that of pure ZnO and binary photocatalysts such as Ag–ZnO and GQDs–ZnO. We believe that this method will lead to the “green” synthesis of hybrid metal/carbon/semiconductor photocatalysts with higher photocatalytic activities.

•We produced the graphite quantum dots (GQDs) by an electrochemical method.•We changed the applied potentials of cycling voltammetry (CV).•Varying of applied potentials changed surface oxygen-containing groups of GQDs.•Higher surface oxidation degree resulted in the red-shift of PL spectra.Graphitic quantum dots (GQDs), as a new class of quantum dots, possess unique properties. Among the various reported approaches for their fabrication, electrochemical method possesses numerous advantages compared with others. In particular, the formation process of the GQDs could be precisely controlled by this method through adjusting the electrochemical parameters and environment. In this study, GQDs with multi-color fluorescence (FL) were obtained by this method through tuning only the applied potential window of cycling voltammetry. The luminescence mechanism of those GQDs was discussed and explained by the ultraviolet (UV)–visible, photoluminescence (PL), and photoluminescence excitation (PLE) spectra. The influence of the applied potential window on the PL properties of GQDs and the relationship between the degree of surface oxidation and PL properties were also investigated.

Herein, we reported a simple and “green” method for preparing the ternary photocatalyst Ag–graphene quantum dots (GQDs)–ZnO. In this method, an aqueous solution of GQDs not only acted as a substituent for the organic solvent for preparing the ZnO precursor but was also used as a reducing agent for the in situ synthesis of Ag nanoparticles (NPs). X-ray diffraction analysis and scanning electron microscopy were employed to confirm the effects of the GQD solution as a solvent on the ZnO structure. Transmission electron microscopy confirmed the synthesis of Ag NPs in the GQD solution as well as the formation of close interconnections between them. Furthermore, photocatalytic tests involving the degradation of Rhodamine B showed that the synthesized ternary photocatalyst displayed excellent visible-light photocatalytic activity, which was much higher than that of pure ZnO and binary photocatalysts such as Ag–ZnO and GQDs–ZnO. We believe that this method will lead to the “green” synthesis of hybrid metal/carbon/semiconductor photocatalysts with higher photocatalytic activities.

In this work, phosphorus-doped graphene quantum dots (P-GQDs) with a high phosphorus doping content (>7 at%) are synthesized via an electrochemical approach. Sodium phytate (C6H6Na12O24P6), a green food antioxidant additive, is used as the electrolyte for providing both a phosphorus source and an electrolysis environment. The obtained P-GQDs exhibit excellent scavenging activity of free radicals, such as hydroxyl radicals (˙OH) and 2,2-diphenyl-1-picrylhydrazyl (DPPH). Combined with Raman, FT-IR, and XPS spectral analyses, the reason for high phosphorus content and the mechanism of free radical scavenging of P-GQDs are investigated in our work.